This application claims priority to an application entitled xe2x80x9cChannel Spreading Device and Method in CDMA Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Dec. 8, 1998 and assigned Serial No. 98-54296, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a channel spreading device and method in a CDMA (Code Division Multiple Access) communication system, and in particular, to a device and method for spreading a channel signal using a Walsh code.
2. Description of the Related Art
As one way to increase system capacity in a CDMA communication system, channelization is provided by use of orthogonal codes. The orthogonal codes can be Walsh codes. The orthogonal channelization is applied to a forward link in the IS-95 standard, for example. A reverse link can be orthogonally channelized by time alignment.
Orthogonal channelization is provided to the forward link in an IS-95 communication system. In FIG. 1, W0-W63 denotes orthogonal codes and each channel is distinguished by its assigned orthogonal code. The orthogonal codes W0-W63 can be Walsh codes. Each channel on the IS-95 forward link is convolutionally encoded and a modulator performs BPSK (Bi-Phase Shift Keying) modulation. The bandwidth used is 1.2288 MHz and the data rate is 9.6 kbps in the IS-95 communication system. Thus, 64 channels (=1.2288 M/(9.6 kxc3x972)) on an IS-95/IS-95A forward link are distinguished by the 64 orthogonal codes W0-W63, as shown in FIG. 1.
The number of available orthogonal codes is obtained after the modulation scheme and the minimum data rate is determined. Future CDMA communication systems will improve system performance by increasing the number of channels available to users.
However, the above IS-95 scheme limits the number of available channels, due to the limited number of Walsh codes available. Consequently, the capacity of channels available to users is limited. It is preferable to use a variable data rate and quasi-orthogonal codes due to their minimal interference with orthogonal codes.
The structure and generation of the quasi-orthogonal codes is disclosed in detail in Korea Application No. 97-47457. The application is for BPSK modulation and sequences have a correlation value of 2m+1( greater than {square root over (L)}) for an odd power of length 2, L=22m+1. A complex quasi-orthogonal function for QPSK (Quadrature Phase Shift Keying) modulation is described in detail in Korea Application No. 98-37453. The complex quasi-orthogonal function is excellent in terms of a correlation value since a correlation value is given {square root over (L)} for L=22m+1, thereby overcoming the correlation value-related problem of quasi-orthogonal functions in BPSK modulation.
In IMT-2000 systems, QPSK modulation is implemented to utilize the above complex quasi-orthogonal function. The resulting QPSK modulation of Walsh codes makes it impossible to achieve backward compatibility between an IMT-2000 system and an existing IS-95 system that employs BPSK modulation to spread specific common channels such as a pilot channel or a sync channel.
The incompatibility between the conventional IS-95 CDMA communication system and the IMT-2000 CDMA communication system will be described in detail. In the following description, the orthogonal code index k, which is applied to the orthogonal code spreader/despreader, is an index used for generating a specific Walsh code and thus the orthogonal code spreader/despreader is a Walsh code modulator/demodulator.
FIG. 2 is a block diagram of a spreading device in a base station using QPSK modulation according to a preferred embodiment of the present invention.
Referring to FIG. 2, after channel encoding, rate matching, and interleaving, odd data aI and even data aQ are applied to the input of signal mappers 211 and 213, respectively. The signal mapper 211 converts 0s and 1s of the odd data aI to +1s and xe2x88x921s, respectively, and outputs the converted data as dI. The signal mapper 213 converts 0s and 1s of the even data aQ to +1s and xe2x88x921s, respectively and outputs the converted data as dQ. An orthogonal code spreader 215 receives the signals dI and dQ from the signal mappers 211 and 213 and an orthogonal code index k, multiplies the signals dI and dQ by the Walsh code Wk corresponding to the orthogonal code index k, and outputs signals XI and XQ [XI+j XQ=(dI+jdQ)*(Wk+jWk)].
A PN code generator 217 generates PN codes PNI and PNQ for spectrum-spreading the orthogonally spread signals XI and XQ. Here, the PN codes can be short PN sequences. A PN masking portion 219 generates spread spectrum signals YI and YQ by multiplying the orthogonally spread signals XI and XQ by their corresponding PN codes PNI and PNQ [YI+jYQ=(PNI+jPNQ)*(XI+jXQ)]. Baseband filters 221 and 223 baseband-pass-filter the spread spectrum signals YI and YQ, respectively. A mixer 225 converts the output of the baseband filter 221 to an RF signal by multiplying it by a carrier cos 2xcfx80fct and a mixer 227 converts the output of the baseband filter 223 to an RF signal by multiplying it by a carrier sin 2xcfx80fct. An adder 229 sums the outputs of the mixers 225 and 227 and outputs the sum as a transmission signal.
As shown in FIG. 2, the signal mappers 211 and 213 convert the signals aI and aQ having 0s and 1s to the signals dI and dQ having 1s and xe2x88x921s, respectively. The orthogonal code spreader 215 receives the orthogonal code index k as well as the signals dI and dQ to orthogonally spread the signals dI and dQ. The signals dI and dQ can be expressed as a complex number dI+jdQ, which is complex multiplied by the Walsh code in its complex form Wk+jWk. This multiplication, which results in XI+jXQ(=(dI+jdQ)*(Wk+jWk)), occurs N times (N is the number of chips in the Walsh code).
FIG. 3 is a block diagram of a mobile station receiver for receiving and demodulating a signal from the base station transmitter shown in FIG. 2 according to a preferred embodiment of the present invention.
Referring to FIG. 3, a mixer 311 mixes a received signal with the carrier cos 2xcfx80fct and a mixer 313 mixes the received signal with the carrier sin 2xcfx80fct. Baseband filters 315 and 317 baseband-pass-filter the outputs of the mixers 311 and 313.
A PN code generator 318 generates the PN codes PNI and PNQ for despreading the received signal. A PN masking portion 319 generates the despread signals XI and XQ by multiplying the signals YI and YQ received from the baseband filters 315 and 317 by the complex conjugate of PN codes PNI and PNQ [XI+jXQ=(PNIxe2x88x92jPNQ)*(YI+jYQ)]. An orthogonal code despreader 321 receives the despread signals XI and XQ and the orthogonal code index k and generates the despread channel signals dI and dQ by multiplying the signals XI and XQ by the complex conjugate of the orthogonal code Wk corresponding to orthogonal code index k [2*(dI+jdQ)=xcexa3(XI+jXQ)*(Wkxe2x88x92jWk)]. A signal mapper 323 converts +1s and xe2x88x921s of the signal dI received from the orthogonal code despreader 321 to 0s and 1s, respectively. A signal mapper 325 converts +1s and xe2x88x921s of the signal dQ received from the orthogonal code despreader 321 to 0s and 1s, respectively. The output signals of the signal mappers 323 and 325 are applied to a combiner (not shown) for use as a channel estimation signal.
In FIG. 3, the PN masking portion 319 and the orthogonal code despreader 321 form a single finger. To estimate channels, the mobile station receiver is provided with a plurality of such fingers.
In the despreading operation of the mobile station, the signals XI and XQ output from the PN masking portion 319 and then the orthogonal code index k are applied to the input of the orthogonal code despreader 321. Here, the orthogonal code index k is known to both the mobile station and the base station. The signals XI and XQ can be represented as the complex number XI+jXQ, which is multiplied by the complex conjugate Wkxe2x88x92jWk of the orthogonal code Wk expressed as the complex number Wk+jWk. Calculated values obtained by performing this operation N times are accumulated to a value twice as great as the input value in the modulation operation of FIG. 2. Therefore, the despreader outputs the accumulated value. If N is 1 in the demodulation, the relation between an input and an output is
xc2xd(dI+jdQ)(Wkxe2x88x92jWk)=xc2xd(dI+jdQ)(Wk+jWk)(Wkxe2x88x92jWk)=(dI+jdQ)xe2x80x83xe2x80x83(1)
FIG. 4 is a block diagram of a base station spreading device using an orthogonal code and BPSK modulation in a CDMA mobile communication system. The spreading device of FIG. 4 is the same as that of FIG. 2 in configuration, except for an orthogonal code spreader 400, the signal mapper 211, and the PN masking portion 219. The orthogonal code spreader 400 spreads a channel signal in BPSK.
Referring to FIG. 4, an input signal a having 0s and 1s is applied to the input of the signal mapper 211 and converted to a signal d having 1s and xe2x88x921s. The orthogonal code spreader 400 receives signal d and orthogonal code index k for orthogonal spreading and outputs d*Wk N times.
FIG. 5 is a block diagram of a mobile station receiver for receiving and modulating a spread signal from the base station transmitter shown in FIG. 4. The mobile station receiver of FIG. 5 is the same as that of FIG. 3 in configuration, except for an orthogonal code despreader 500 which performs channel despreading in BPSK.
Referring to FIG. 5, the orthogonal code despreader 500 receives signal X from the PN masking portion 319 and the orthogonal code index k. The orthogonal code index k is known to both the mobile station and the base station. The signal X is multiplied by the Walsh code Wk used in the base station. Calculated values obtained by performing this operation N times are accumulated to a value twice as great as the input value in the modulation operation of FIG. 4. Therefore, the orthogonal code despreader 500 outputs the accumulated value. If N is 1 in the demodulation, the relation between an input and an output is
xc2xd(dI+jdQ)Wk=xc2xd(dI+jdQ)WkWk=(dI+jdQ)xe2x80x83xe2x80x83(2)
The IS-95 system employs a BPSK orthogonal spreading scheme, whereas the IMT-2000 system may use a QPSK orthogonal spreading scheme. In this case, it is impossible to conduct communications between a base station of the IMT-2000 system and a mobile station of the IS-95 system and between a base station of the IS-95 system and a mobile station of the IMT-2000 system.
In order to describe the problem, it is assumed that the base station in the IMT-2000 system subjects a signal to QPSK modulation and the mobile station in the IS-95 system subjects a modulated signal to BPSK demodulation. Therefore, when the base station transmits a QPSK modulation signal modulated as shown in FIG. 2 and the mobile station despreads a spread channel signal in BPSK as shown in FIG. 5, the relation between an input value and an output value of the demodulator is
xc2xd(XI+jXQ)Wk=xc2xd(dI+jdQ)(Wk+jWk)Wk=(dIxe2x88x92jdQ)+j(dI+jdQ)xe2x80x83xe2x80x83(3)
It is noted from Eq. 3 that not the original signal SI+jSQ, but (dIxe2x88x92jdQ)+j(dI+jdQ) is output from the demodulator on the above assumption. Due to the difference between BPSK modulation input and QPSK demodulation output, the base station cannot communicate with the mobile station. This also applies to the reverse case where the base station spreads a channel signal in BPSK and the mobile station demodulates a BPSK modulation signal in QPSK.
To solve this problem, Korea Application No. 98-49863 suggests a base station transmitter for IMT-2000 which can perform both BPSK and QPSK orthogonal spreading. The base station transmitter spreads a common channel signal (pilot channel, sync channel, and paging channel) used in an IS-95 base station in BPSK modulation and the other channels (dedicated channels) in BPSK or QPSK depending on the reception scheme of a mobile station communicating with the base station. In contrast, the present invention applies one QPSK orthogonal modulation scheme to all forward channels, while achieving compatibility with a conventional IS-95 mobile station using a BPSK reception scheme.
An object of the present invention, therefore, is to provide a channel signal transmitting/receiving device having a QPSK channel spreader and a BPSK receiver and a method thereof in a CDMA communication system.
Another object of the present invention is to provide a channel signal transmitting/receiving device having a BPSK channel. spreader and a QPSK receiver and a method thereof in a CDMA communication system.
A further object of the present invention is to provide a device and method for enabling a base station to transmit a QPSK spread channel signal and a mobile station to despread the QPSK spread channel signal by use of a BPSK channel despreader in a CDMA communication system.
Still another object of the present invention is to provide a device and method for enabling a base station to transmit a BPSK spread channel signal and a mobile station to despread the. BPSK spread channel signal by use of a QPSK channel despreader in a CDMA communication system.
These and other objects are achieved by providing a demodulation method in a mobile station having a plurality of channels, for receiving a signal from a base station. In a first embodiment of the present invention, the base station has a plurality of channels, a BPSK spreader for orthogonally spreading symbol data of each channel with an assigned orthogonal code, and a QPSK spreader for PN-spreading the orthogonally spread signal with a PN code. In the demodulation method, a first QPSK despreader in the mobile station receives PN-spread signal and PN-despreads the PN-spread signal with the PN code, and a second QPSK despreader orthogonally despreads the PN-despread signal with a complex orthogonal code having the real and imaginary parts of the assigned orthogonal code. During the orthogonal despreading, the complex conjugate of the estimated pilot channel value is multiplied by the orthogonally despread signal, for compensation.
In a second embodiment of the present invention, the base station has a plurality of channels, and is comprised of a QPSK spreader for spreading symbol data of each channel by the real and imaginary parts of its assigned orthogonal code, and a QPSK spreader for PN-spreading the orthogonally spread signal with a PN code. In the demodulation method, a mobile station receives the PN-spread signal from the base station, PN-despreads the received signal with a PN code by a QPSK despreader, and orthogonally despreads the PN-despread signal with the assigned orthogonal code by a BPSK despreader. During the orthogonal despreading, the complex conjugate of the estimated pilot channel value is multiplied by the orthogonally despread signal, for compensation.